An X-ray search for active cores in Local Group galaxies

Abstract

The nuclei of galaxies are sometimes sites of very energetic process, resulting in high levels of electromagnetic radiation being emitted. Such active galactic nuclei (AGNs) are thought to be powered by accretion of mass onto a centrally collapsed object, a supermassive black hole of typically 10® - 10® solar masses. When gravitational energy of the accreting material is liberated it is radiated away, causing the nucleus to appear very bright. The extreme manifestation of this phenomenon is probably seen in the famous Quasi Stellar Objects. In this thesis, a dedicated search for X-ray emission from the cores in the Local Group galaxies is performed to establish whether low levels of nuclear activity in these nearest galaxies may be found. The data are from ROSAT HRI or PSPC pointing observations which are available for 21 Local Group galaxies in the centre of the observing fields, and for another 3 in the outer area, covering 70% of all Local Group members. The HRI data are particular useful, since the nuclear source may be better resolved from the HRI images.